Non-consumable water is in abundance, but clean water is not. Clean water is vital to humans and has been described as the Oil of the 21st Century. Clean is defined here as pure enough to be used for irrigation and watering livestock. The world is becoming more populous, and the water needed to sustain and feed that population is becoming scarce. In response to this imminent clean-water crisis, governments are tightening both drinking water and wastewater regulations. The natural result of this increase in treatment requirements is an increase in treatment costs.
Distillation has long been used to purify liquids, extract solutes, and concentrate substances from solution. The three most common distillation technologies are: multiple effect evaporation, multistage flash evaporation, and vapor-compression distillation.
As used herein, “distillation” is the volatilization or evaporation and subsequent condensation of a liquid solvent for the purpose of separating the solvent from the solution. Distillation is used in many industries, from oil refineries that use distillation to acquire gasoline from crude oil, to alcoholic beverage makers that use distillation to extract alcohol from mash. Historically, distillation was rarely used for treating water because the cost of distillation exceeded the cost of other, more traditional, forms of treatment.
The process of distillation is typically performed by adding heat to the solution (influent) at atmospheric pressure until the solvent (water) vaporizes. Other traditional methods use high pressure pumps and nozzles to create pressure gradients. Still other methods involve refrigeration units to remove energy from the system to freeze portions of the influent in order to create ice crystals. Still other systems use electrolysis to produce water vapor from water.
The Achilles heel of these distillation processes is the quantity of energy necessary to convert liquid water to a vapor. Additionally, these common technologies require pretreatment of the influent which makes the process even more expensive. And, these traditional methods generate effluents having characteristics that depend directly on the particular type of influent used. For example, feedwater supplied to the unit may contain volatile organic compounds (VOCs).
In addition, as a wastewater process, conventional distillation may not meet the discharge requirements for dissolved oxygen content, pH, temperature, and VOCs. The effluent from landfills is likely to have the highest concentration of VOCs. Given this reality, landfill effluent should preferably be pretreated with aeration prior to distillation or double-distilled to produce a higher quality distillate. The additional costs incurred with this step may be offset by the use of landfill gas as a supplementary energy (heat) source.
Minimum dissolved oxygen levels are developed by the authority having jurisdiction on a case-by-case basis. According to the EPA, typical values are between 5 and 7 mg/L.
According to the American Water Works Association, Inc., the target pH for potable water to protect from copper and lead corrosion is 8.0 to 8.5. The pH of distilled water is neutral (7.0) at distillation but, according to the EPA, may quickly become acidic due to CO2 gas dissolving into the distilled water from the air.
Similarly, some jurisdictions have discharge temperature requirements that are specific to the stream and to the flora and fauna within the stream.
If one is to use distillation as part of a drinking water treatment process, VOCs, pH, fluoridation, and chlorination may need to be addressed prior to distribution.